通过此简单代码，我可以在屏幕底部显示对话框，如以下

但是我有三个简单的问题：

1. 在对话框底部（例如20.0在显示对话框上）设置边距
2. .reverse()上解雇对话框
3. 单击对话框外部以关闭对话框

完整的源代码：

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  void showPopup() {
    showDialog(
      context: context,
      builder: (_) => PopUp(),
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: showPopup,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}

class PopUp extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => PopUpState();
}

class PopUpState extends State<PopUp> with SingleTickerProviderStateMixin {
  AnimationController controller;
  Animation<double> opacityAnimation;
  Tween<double> opacityTween = Tween<double>(begin: 0.0, end: 1.0);
  Tween<double> marginTopTween = Tween<double>(begin: 300, end: 280);
  Animation<double> marginTopAnimation;

  @override
  void initState() {
    super.initState();

    controller = AnimationController(duration: const Duration(milliseconds: 300), vsync: this);
    marginTopAnimation = marginTopTween.animate(controller)
      ..addListener(() {
        setState(() {});
      });
    controller.forward();
  }

  @override
  Widget build(BuildContext context) {
    return FadeTransition(
      opacity: opacityTween.animate(controller),
      child: Material(
        color: Colors.transparent,
        child: Container(
          margin: EdgeInsets.only(
            top: marginTopAnimation.value,
            left:20.0,
            right:20.0,
          ),
          color: Colors.red,
          child: Text("Container"),
        ),
      ),
    );
  }

  @override
  void dispose() {
    controller.dispose();
    super.dispose();
  }
}